This invention pertains generally to precursors and deposition methods for low dielectric constant thin films on semiconductor substrates, and more particularly to such precursors and deposition methods suited to aerogel thin film fabrication.
Sol-gel techniques are commonly used to produce dense thin films in semiconductor fabrication. The word sol-gel, however, does not describe a product but a reaction mechanism whereby a sol may be transformed into a gel. A sol is a colloidal suspension of solid particles in a liquid; one method of forming a sol is through hydrolysis and condensation reactions which cause a multifunctional monomer in a solution to polymerize into relatively large, highly branched particles.
Many monomers suitable for polymerization are metal alkoxides. For example, a tetraethylorthosilicate (TEOS) monomer may be partially hydrolyzed in water by the reaction
Si(OEt)4+H2Oxe2x86x92HOxe2x80x94Si(OEt)3+EtOH
Reaction conditions may be controlled such that, on the average, each monomer undergoes a desired number of hydrolysis reactions to partially or fully hydrolyze the monomer; TEOS which has been fully hydrolyzed becomes Si(OH)4. Once a molecule has been at least partially hydrolyzed, two molecules can then link together in a condensation reaction, such as
(OEt)3Sixe2x80x94OH+HOxe2x80x94Si(OH)3xe2x86x92(OEt)3Sixe2x80x94Oxe2x80x94Si(OH)3+H2O
or
(OEt)3Sixe2x80x94OEt+HOxe2x80x94Si(OEt)3xe2x86x92(OEt)3Sixe2x80x94Oxe2x80x94Si(OEt)3+EtOH
to form an oligomer and liberate a molecule of water or ethanol. The Sixe2x80x94Oxe2x80x94Si configuration in the oligomer formed by these reactions has three sites available at each end for further hydrolysis and condensation; thus additional monomers or oligomers can be added to this molecule in a somewhat random fashion to create a highly branched polymeric molecule from literally thousands of monomers.
Through continued reactions, one molecule in the sol may eventually reach macroscopic dimensions so that it forms a network which extends throughout the sol; at this point (called the gel point), the substance is said to be a gel. By this definition, a gel is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. As the skeleton is porous, a gel can also be described as an open-pored solid structure enclosing a pore fluid. An oligomerized metal alkoxide, as defined herein, comprises molecules formed from at least two alkoxide monomers, but does not comprise a gel.
An ungelled sol may be dip-coated or spin-coated onto a substrate to form a thin film on the order of several microns or less in thickness, gelled, and dried. In practice, such a thin film is subjected to rapid evaporation of volatile components, to the extent that the deposition, gelation, and drying phases may all be taking place at once as the film collapses rapidly to a dense film. Drying by evaporation of the pore fluid produces extreme capillary pressure in the microscopic pores of the wet gel, causing many pores to collapse and the gel to be reduced in volume as it dries, typically by an order of magnitude or more.
A dried gel which is formed by collapsing and densifying a wet gel during drying is termed a xerogel. A thin film xerogel is usually dense, with just a few percent porosity remaining after drying. U.S. patent application Ser. #08/247,195 now U.S. Pat. No. 5,470,802, to Gnade, Cho and Smith discloses a process for producing an aerogel thin film on a semiconductor substrate; an aerogel thin film is distinguishable from a xerogel thin film primarily by a manner of drying which largely avoids pore collapse during drying of the wet gel; this results in a substantially undensified thin film which can be fabricated with almost any desired porosity (thin films with greater than 90% porosity have been demonstrated). Such films have been found to be desirable for a low dielectric constant insulation layer in microelectronic applications.
The present invention provides an aerogel precursor sol and a method for deposition of aerogel thin films, e.g. for microelectronic applications. For such applications, the precise control of film thickness and aerogel density are desirable. Several important properties of the film are related to the aerogel density, including mechanical strength, pore size and dielectric constant. It has now been found that both aerogel density and film thickness are related to the viscosity of the sol at the time it is spun onto a wafer; this presents a problem which was heretofore unrecognized, the problem being that with conventional precursor sols and deposition methods, it is extremely difficult to control both aerogel density and film thickness independently and accurately.
Aerogel thin films may be deposited on patterned wafers, e.g. over a level of patterned conductors. It has now been recognized that sol deposition should be completed prior to the onset of gelation to insure that gaps between such conductors remain adequately filled and that the surface of the gel remains substantially planar. To this end, it is also desirable that no significant evaporation of pore fluid occur during gelation. Unfortunately, it is also desirable that the gel point be reachable as soon after deposition as possible to simplify processing, and the conventional method for speeding gelation of thin films is to allow evaporation to occur. It is recognized herein that a suitable precursor sol for aerogel deposition should allow control of film thickness, aerogel density, gap fill and planarity, and be relatively stable prior to deposition, and yet gel relatively soon after deposition without substantial evaporation.
A method has now been found which allows controlled deposition of aerogel thin films from a multi-solvent precursor sol. In this method, sol viscosity and film thickness may be controlled relatively independently. This allows film thickness to be rapidly changed from a first known value to a second known value which can be set by solvent ratios and spin conditions, thus keeping film thickness largely independent of aerogel density and allowing rapid gelation. However, at the same time, the solid:liquid ratio present in the film at the gel point (and therefore the aerogel density) can be accurately determined in the precursor sol prior to deposition, independent of spin conditions and film thickness.
An aerogel precursor sol particularly suited for microelectronic thin film fabrication is disclosed herein. The aerogel precursor sol comprises an oligomerized metal alkoxide, particulate or colloidal metal oxides, and/or organic precursor dispersed in a high volatility solvent and a low volatility solvent, the high volatility solvent having a vapor pressure higher than the vapor pressure of the low volatility solvent. In this sol, the low volatility solvent to metal alkoxide ratio preferably is set to a value capable of forming a gel of a desired density. The high volatility solvent preferably has a ratio to low volatility solvent required to maintain a specified viscosity for the precursor sol.
The high volatility solvent is preferably selected from the group consisting of methanol, ethanol, ketones and combinations thereof. As the low volatility solvent generally comprises at least some water, it is preferable that the low volatility components be water miscible. The low volatility solvent is preferably selected from the group consisting of water, 1-propanol, 2-propanol, 1-butanol, 2-butanol, tert-butanol, 2-pentanol, 3-pentanol and combinations thereof.
In another aspect of the invention, a method of forming a porous dielectric on a semiconductor substrate is disclosed. This method may comprise the step of depositing a thin film of an aerogel precursor sol on a semiconductor substrate; the sol comprises an oligomerized metal alkoxide dispersed in a first solvent and a second solvent. The method further comprises preferentially evaporating substantially all of the first solvent from the thin film, preferably without substantial evaporation of the second solvent, and subsequently cross-linking the thin film to form a wet gel having pores arranged in an open-pored structure on the semiconductor substrate. Preferably, the first solvent has a vapor pressure which is higher than the vapor pressure of the second solvent such that it may be removed via differential evaporation rate; this rate may be enhanced by performing the preferential evaporation step in a controlled atmosphere having a partial pressure of the second solvent.